Synthetic polymers are used in a wide variety of applications. In many applications, these synthetic polymers are crosslinked in order to achieve the required performance properties. For over sixty (60) years, a large class of commercially important thermoset polymers has utilized formaldehyde-based crosslinking agents. Such crosslinking agents based on formaldehyde traditionally have provided an efficient and cost-effective means of curing a wide variety of materials. Examples of formaldehyde-based crosslinking agents include melamine-formaldehyde, urea-formaldehyde, phenol-formaldehyde and acrylamide-formaldehyde adducts. With growing toxicity and environmental concerns, there has been an ongoing search to replace formaldehyde-based crosslinking systems. However, these alternative systems have suffered from significant deficiencies including low or slow cure, requiring end users to change their commercial high speed application equipment, and emission of toxic components or volatile organic compounds other than formaldehyde.
A particularly attractive crosslinking reaction involves the esterification of a carboxylic acid functional polymer with a hydroxyl functional molecule. This is an attractive reaction since water is the only product of the crosslinking reaction. In EP 583 086 A1, for example, small molecule polyol or alkanolamine crosslinkers are cited as crosslinkers for carboxyl-functional polymers. However, this reaction is difficult to conduct effectively since it is slow under conventional time and temperature conditions of cure.
Outside of the investigations surrounding non-formaldehyde crosslinking agents, graft copolymers of starch and acrylic acid have been described in the literature. Graft copolymers of starch and acrylic acid have been used as water absorbing agents. Also, starch-stabilized binder compositions containing starch-polymer grafts derived from low molecular weight starch hydrolyzates have been described.
To address the need for a formaldehyde-free crosslinking system, we have found unexpectedly that selected polysaccharides in combination with selected polycarboxylated polymers provide a safe and facile crosslinking system, as described herein below.